A typical laser diode array has a plurality of laser diode active regions formed on a common substrate. Generally, a quantum well layer is disposed on the substrate and the active regions in the quantum well layer may be optically isolated from each other by various techniques. Electrical connection may be made to each of the active regions individually for an addressable array or to all regions simultaneously for phase coupled laser diode array having a continuous wave high power output (the output power of each individual active region being combined with each other active region).
The optical isolation between each active region in the quantum well layer is required to prevent lateral coupling of optical energy between adjacent active regions. Such lateral coupling may cause unwanted oscillations and reduce the optical power output of the device in the phase coupled array. In an addressable array, the lateral coupling may cause crosstalk. Such crosstalk could introduce bit errors where the array is used for fiber optic communications. Accordingly, several techniques isolating the active regions of the quantum well layer in the laser diode array have been developed in the prior art.
For example, Thornton, U.S. Pat. No. 4,870,652 and Paoli, et al., U.S. Pat. No. 4,831,629 show that the use of diffused regions which extend into the multiple quantum well layer will provide lateral isolation between adjacent laser diode active regions. The diffused regions will absorb laterally propagating optical energy from the active regions to minimize crosstalk and undesired transverse modes.
Another technique developed in the prior art is the use of a plurality of parallel etched grooves extending in the longitudinal direction through the quantum well layer, as disclosed in Scifres, et al., U.S. Pat. No. 4,803,691. The grooves reflect and scatter the laterally propagating light such that the optical coupling between adjacent active regions is minimized.
Instead of diffused regions or scattering grooves to separate the active regions, an absorbent material may also be used to provide optical isolation. The absorbent material may be any of various semiconductor materials such as silicon or the exemplary III-V semiconductor compounds as disclosed in Luft, U.S. Pat. No. 4,712,220. One technique for placing absorbent material in between the active regions of the laser diode array is to etch the quantum well layer to form voids between the active regions. The absorbent semiconductor material is then grown within the voids between the active regions. For example, Trussel, et al., U.S. Pat. No. 4,371,968 discloses the regrowth of gallium aluminum arsenide within trenches formed between the active regions. Carney et al., U.S. Pat. No. 4,577,321 discloses the growth of cladding material to fill the voids between individual quantum well lasers. An overview of all the above techniques disclosed in Heinen et al., U.S. Pat. No. 4,674,095.
A significant disadvantage and limitation of each of the above described techniques for providing isolation between the quantum well layer active regions is that several additional processing steps are required to provide for the diffusion into the quantum well layer or for the regrowth of material within etched voids. The diffusion or regrowth must be performed at relatively high temperatures which causes dopant carriers within the device to further diffuse. This further diffusion may degrade device operating characteristics from the desired characteristics which had been established during the growing of the quantum well layer. Also, the additional processing steps increase wafer handling time and complexity of the device, each of which significantly adds to cost of the finished array. Accordingly, it would be desirable to provide for optical isolation within the quantum well layer at the time such layer is formed.